Lawnmowers setting off dropped rounds -
For several decades I maintained a private property that had a range on it. Most casings were left here they fell into the turf, and more than a few live rounds were dropped a lost. The shooting area , along with the rest of the property , was maintained to a high standard and mowed 1-2 times per week. Mower setting was 3". Never once did a round get set off by the mower , and I can say that there was not even any evidence of kicking a spent casing out of the mower deck. At times the area was mowed short and tight as a croquet court , mowing level down to 1 1/2 - 1 1/4" , and that is short by turf standards. (golf course fairways are about 1".) No casing strikes. The metal casings are fairly heavy and lay low in the turf crown. It should be noted that this was an occasional range , not daily high traffic. But - I do remember several occasions in which many hundreds of rounds were discharged in a session , and I mowed within a few days.
For a round to struck by a mower blade that mower would have to be set really low , almost scalp level.